Electrocatalytic Ammonia Oxidation Mediated by Nickel and Copper Crystallites Decorated with Platinum Nanoparticle (PtM/G, M = Cu, Ni)

The nickel (Ni) and copper (Cu) oxides decorated with Pt nanoparticles (Pt NPs) were synthesized via electrodeposition for direct ammonia electro-oxidation. Voltammetry results revealed the reversible oxidation states of the transition metals as mediating the electron transfer of nitrogen species. Cu and Ni oxides showed catalytic ammonia oxidation at working potentials of +0.23 V and +1.1 V (vs Hg/HgO), respectively, and the peak current was significantly improved by Pt modification. In a constant current mode, the two-electrode electrolysis determined the efficiency of NH3 conversion to NO3- and N2. At a current density of 1.5 mA cm-2, the promising removal efficiency of NH3 (>90%) on Cu and Ni oxides was attributed to the high electrochemically effective surface area (AECSA). The reaction mechanism of NH3 oxidation and production of different nitrogen species were evaluated via a steady-state approach. The decoration of Pt NPs onto PtM/G and a low applied current favored ammonia conversion more to nitrogen gas than the monometal electrode did. Pt sites enhanced the dehydrogenation of NH3, thus moderately increasing the N2 selectivity up to 60% on the PtCu/G electrode for an initial NH3 of 50 mg-N L-1.